Nitrogen-utilization efficiency during early deficiency after a luxury consumption is improved by sustaining nitrate reductase activity and photosynthesis in cotton plants

Aims: Understanding mechanisms underlying N use efficiency (NUE) after luxury consumption and nitrate deprivation is crucial to crop productivity. The aim was to elucidate the importance of photosynthesis, assimilatory nitrate reduction and N-reserve remobilization to NUE in cotton. Methods: Plants were exposed to three conditions in nutrient solution: (a) previous exposure to high nitrate supply (10 mM) for long-term (8 days); (b) nitrate deprivation (NO 3 − withdrawal) for 8 days followed by (c) an early N-deficiency for 4 days. Results: Plants supplied with nitrate excess were able to display increment in shoot NUE related to dry matter gain, whereas photosynthetic N use efficiency did not change, evidencing that excess N per se was not able to improve CO 2 assimilation. Nitrate reductase (NR) activity was crucial to remobilize stored nitrate through deprivation phase and free amino acids, total proteins, and chlorophylls were also essential to N-remobilization. High NUE was important to kept high root growth rates throughout deprivation and early deficiency phases. Despite the great decrease in chlorophyll content, PSII and PSI activities were kept stable until the onset of early N-deficiency, when cotton plants displayed high shoot NUE. Conclusions: These responses are closely associated with high NR activity and sustaining of photosynthesis, which contribute to N-homeostasis in different nutritional regimes.